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Why do the laws of quantum mechanics apply to subatomic particles and not to large objects?

Here’s the short answer: The laws of quantum mechanics apply to things that are in a quantum state, that is, in many different states at the same time (a superposition of states). For example, a photon in a quantum state might be in the state of being in Position A, B, and C at the same time. The laws of classical physics apply only to things that are in an ordinary state—in this example, occupying only one position at a time. Let’s say that the photon hits the retina of a frog and is absorbed by a chemical in the retina. Let’s say that the retina is Position C. So, the photon has lost its superposition and for a brief moment occupies Position C.

I’ve heard that frogs have such good night vision that they can see a single photon. When the photon is absorbed by the electron in the frog’s retina, the frog might see a little spark of light. Now, the photon has left the quantum world of being in a superposition of many states and joined the macroscopic world, in which it’s in a single state. Its behavior is now understood according to the classical laws of physics, in particular, the laws of electromagnetism.

Macroscopic objects are not subject to the laws of quantum mechanics because they cannot maintain the quantum state of being in many states at the same time. Their particles are close together and interact with each other and with things in the environment like air molecules. Interaction causes their particles to lose their quantum states and reduce themselves to occupying single states. Such objects follow the laws of classical physics.

This explanation is based on the principle of decoherence. When the particles of macroscopic objects drop out of their quantum states (considered a “coherent” state), they are said to “decohere.”

That was the short answer.

A note. Before explaining more, I’d like to note that both subatomic particles and large objects follow the laws of relativity with one major exception. You’re probably referring to this exception: According to the laws of quantum mechanics, a subatomic particle correlates its behavior (such as spin) with an entangled particle instantaneously, regardless of distance. This violates the principle in Special Relativity that nothing can travel faster than the speed of light. Otherwise, subatomic particles, like macroscopic objects, follow the laws of relativity, like experiencing longer times when hitting greater relative speeds, etc.

More explanation. Getting back to your question…the answer as to why two different sets of laws apply depends upon the interpretation of quantum mechanics. I’ve summarized the decoherence interpretation developed in the 1970’s-1990’s. The original interpretation, the Copenhagen Interpretation, developed in the 1920’s, provides no answer at all. Instead, it maintains that such questions should not be asked as they cannot, in principle, be answered within physics. The Copenhagen interpretation eventually devolved into an interpretation that is popular to this day, “Shut up and calculate!”

I find the Transactional Interpretation helpful in understanding the physical meaning of what is happening in decoherence. The Transactional Interpretation was developed in the 1980’s by Dr. John Cramer and further developed recently by Dr. Ruth Kastner (as described in lay terms in “Understanding Our Hidden Reality, Solving Quantum Riddle). The following description is based on this interpretation.

When a particle, like our photon, is in a superposition, it is in a condition of manifesting two or more mutually exclusive properties at the same time. In this case, the photon is in a superposition of being in Positions A, B, or C. A superposition is not a real physical universe condition. In our physical universe, a single particle cannot be located in three different places at the same time. According to the Transactional Interpretation, the photon is operating in a different level of reality from ordinary physical universe reality. Let’s call this realm “Quantumland.” This is the realm to which quantum mechanics applies.

However, once the photon interacts with an electron in the frog’s retina, it enters physical reality. A particle interaction is a real physical universe energy exchange with another particle. The photon has given a bit of energy to an electron in the frog’s retina.

Once energy is exchanged, the universe has recorded a change. This is the creation of information. Now, the universe is different. No one can undo this. The frog saw the spark. The interaction has happened, the physical universe has changed, and the exchange of energy is now part of the past. Classical physics laws apply to physical reality.

As a note, it isn’t necessary for a conscious being like a frog to record the information, as in this case. it’s only necessary that there be an exchange of physical universe energy between particles. That’s sufficient to change the course of the universe, at least in a tiny way. And, it’s sufficient for a particle to lose its quantum state and join into creating the history of the physical universe.

buckyball, molecule with 60 carbon atoms
Buckyball, a molecule comprising 60 carbon atoms. [Image source: By Shmuel Benezra – collaborative work with my kids Agam and David, we have built the model and photographed it. License: CC BY-SA 4.0, https://en.wikipedia.org/wiki/Buckminsterfullerene. Retrieved July 30, 2018.]

There’s no specific maximum size at which particles no longer follow quantum laws, and classical physics laws kick in. Large molecules with as many as 60 carbon atoms have been found to follow the laws of quantum mechanics. These molecules are called buckyballs. At this time, physicists continue to do experiments to determine the maximum size of particles that can maintain quantum states before an interaction within itself, like a release of radiation, or an interaction with another particle kills the quantum state.